Devices of the above mentioned type are used, for example, for pinpointing hot boxes. A device of the known type has been disclosed in European Patent Disclosure EP 41 178 Al, with the aid of which it is possible to detect not only hot boxes, i.e. overheated bearings, but also wheel rims or brakes of moving railroad cars. With these known devices, two separate infrared beam detectors with appropriate representation devices are disposed in a common housing. A further embodiment of this known device is limited to only one infrared beam detector, the measuring beam of which obliquely impinges on the passing wheel at the height of the wheel hub and therefore measures the temperature of the wheel rim or the brake and the bearing temperature sequentially over time. As a rule, an embodiment of this type with only one infrared beam detector is not suitable to detect hot boxes and defective brakes of a wheel by means of a common device. Finally, when differently heated components are detected together by means of only one infrared beam detector it must also be taken into consideration that too great temperature differences, together with a correspondingly rapid sequence of the measuring values, can distort the measured results from measuring points which are different in relation to each other to the point of uselessness. As a rule, threshold switches are used in evaluation circuits, which, with respectively distorted measured values, by their nature do not provide a useful correspondene between the signals and the measuring point and might generate delayed warnings.
A design is disclosed in European Patent Publication EP-A 0 424 570, wherein a plurality of lenses are directed to different positions of passing rail vehicles. To preserve the evaluation electronics, optical wave guides are disposed between the lenses and a common detector, which makes possible an increased spatial separation between the lenses and the evaluation unit.